In a generally-used laser printer, a picture is formed on a photosensitive material by a single light beam modulated by a picture signal. Also known is a multi-beam laser printer in which a plurality of scanning lines are simultaneously formed by a plurality of beams in order to perform high speed picture formation.
FIG. 1 is a diagram showing an example of the conventional multi-beam laser printer. In the drawing, a plurality of beams from a laser diode 1 as one body having a plurality of separated luminous points are irradiated on a rotary polyhedral mirror 23 after the luminous flux of the beams has been made parallel through a collimating lens 4. The plurality of beams are deflected by the rotary polyhedral mirror 23, and irradiated on a photosensitive material 25 through a f.theta. lens 24. A scanning initation detector 26 for detecting the scanning positions of the beams is provided at an end portion of the photosensitive material 25.
The beam spots on a surface of the rotary polyhedral mirror 23 are aligned at right angles to the main scanning direction as shown in FIG. 2. Therefore, the beam spots and loci thereof in multi-beam scanning, that is, scanning lines S, are formed on the photosensitive material 25 as shown in FIG. 3. Those scanning lines S form a picture, and an interval a.sub.1 between adjacent scanning lines represents the resolution of the picture.
In the multi-beam laser printer as described above, the laser diode 1 having a plurality of separated luminous points is effectively used as a light source for supplying a picture output at a high speed in a laser printer or the like, because the laser diode 1 can act as a small-sized light source for generating a plurality of beams.
In the foregoing laser diode 1 having a plurality of separated luminous points, however, the intervals of the luminous points and the size of each luminous point cannot be desirably set because of drawbacks in semiconductor product producing techniques and thermal interference between the luminous points.
Therefore, there has been a limit in the spot size and in the beam interval on an irradiation surface of the photosensitive material 25. Consequently, there has been a limit in picture resolution.
In order to solve this problem, there has been proposed a multi-beam laser printer as disclosed in Japanese Patent Unexamined Publication Nos. 29208/1981 and 67375/1982, in which a laser diode 1 rotated through a fixed angle in the direction of an arrow A shown in FIG. 1 so that the alignment of the beam spots on a surface of a rotary polyhedral mirror 23 is inclined as shown in FIG. 4. By making the alignment of the beam spots inclined as described above, the intervals of the beam spots in the direction perpendicular to the main scanning are as shown in FIG. 5. As a result, the intervals a.sub.2 of the scanning lines S on a photosensitive material 25 is less than the interval a.sub.1 as shown in FIG. 3. The reduced interval a.sub.2 improves the resolution of the picture.
The foregoing conventional technique, however, is disadvantageous in that the rotary polyhedral mirror 23 is enlarged in size, or in that an effective picture width is reduced or a picture is inclined which thereby causes a deterioration of picture quality.
Further, although there has been proposed a technique in which the intervals of the beam spots are reduced by using optical fibers, no multi-beam laser printer using such a proposed technique has been practically used because of a complicated arrangement thereof.